Nearly 4,600 more had been hit with delays as of the same time, according to FlightStats, another flight-tracking service.

That total for both delays and cancellations was likely to grow as the evening progresses, as a long line of thunderstorms, severe winds, areas of poor visibility and even some snow advanced toward some of the nation's busiest - and most delay-prone - airports.

The Federal Aviation Administration's flight delays map also showed long delays at a number of airports as of 4 p.m. ET. A combination of thunderstorms and low clouds were leading to delays ranging from one to three hours at Atlanta, Chicago O'Hare, Philadelphia and at all three of the big New York City-area airports (Newark, LaGuardia and JFK).

Conditions are likely to remain poor into the evening as the cold front pushes into metro areas such as New York, Philadelphia, Washington and Atlanta. And the problems could spread, since severe flight disruptions in the Northeast and Midwest often ripple out and affect other parts of the USA.

A flight from Phoenix to Seattle, for example, could become delayed or canceled if the aircraft or crew schedule to fly it gets bogged down in the stormy East.